1. Field of the Invention
This invention relates to methods of analysis of the production rates and pressures of hydrocarbons from wells, particularly to automated calculation of the formation properties adjacent to the liquid hydrocarbon producing wells.
2. State of the Art
One can determine formation properties near a wellbore and general information about the condition of the wellbore in a liquid hydrocarbon producing well. Pressure transient techniques, the preferred methods for such determinations, measure how pressure in the well bore changes with time. In a commonly used pressure transient technique, the pressure build-up test, the operator shuts the well in and measures pressure recovery as a function of time.
The properties of the well can be determined using the pressure build-up test by shutting the well in and measuring its pressure recovery as a function of time. Ideally, the operator reworks the well down-hole to shut the well in at the top of the producing formation, a procedure hereinafter referred to as "down-hole shut-in." Although this approach would provide quick and accurate results, it requires expensive reworking the well down-hole. Therefore, economics normally dictates the practice of shutting the well in at the surface, herein after "surface shut-in." However, once the operator shuts the well in at the surface, fluid continues to flow from the producing formation into the well past the level of the top of the formation resulting in long test times. The flow, or "afterflow," as it is conventionally called, flows into the well bore and past the top of the formation distorting and prolonging the test. Eventually, the shut-in pressure recovers to the value that would have been recorded in a down-hole shut-in test. Such pressure equalization can take a long time, possibly over 100 hours. Once the pressure build-up data have been obtained, several different kinds of information about the well bore and the formation adjacent the wellbore can be determined, for example, a) formation pressure, b) formation permeability, and c) the well bore condition or damage, called "skin."
Pressure build-up tests are particularly beneficial for analyzing the reasons for the flow rate decline of a slow flowing well. Declines can occur for a variety of reasons, including plugging the perforations in the casing of the well, precipitation of scale in the formation immediately around the well bore, and exhaustion of the amount of hydrocarbon present in the formation. If the reason for the slow flow rate is formation damage, one may stimulate the well by fracturing or by pumping acid into the formation. However, if the reason slow flow is exhaustion of the resource, one may simply choose to abandon the well. Therefore, the operator of a slow flowing well needs to know the reason a well flows slowly. Unfortunately, oil production economics dictate that a slow flowing well may never return the cost of a down-hole shut-in, nor can the operator afford the loss of production that a surface shut-in requires. Consequently, operators only reluctantly test the wells that could benefit the most from the analysis.
Economics aside, there are problems with analyzing data obtained from slow flowing wells. In theory, the early part of a pressure build-up test contains important information concerning the well and the formation. However, in practice, the early data are noisy due to the wellbore afterflow. The early data are more sensitive to inconsequential impediments in the well perforations and the well bore.
Slow flowing wells may flow slowly for several reasons. Corrective action can be taken once the reason for the slow flow is known, but the time and loss of production involved in pressure build-up testing makes operators reluctant to test these wells. The operators of slow flowing wells need a quick pressure build-up test. Such a test would be able to use the early data obtained before pressure equilibration, thereby providing a short, accurate pressure build-up test.